Such methods are used, for instance, for checking betting forms, e.g. pools coupons, for the purpose of reading marks entered manually on the coupons and for the purpose of reading information printed on said coupons in the form of mark locations, coupon identifying data, and like information. Such coupons are normally produced from low-quality paper.
It is highly imperative that such coupons can be read reliably, not least in the case of betting forms. It is also desirable to utilize high detection or sensing sensitivity.
However, many factors influence the detection result and create problems, inter alia in the interpretation of the measurements values obtained. For example, among other things the short beam path will result in uneven imaging, i.e. when reading a smooth, white document the output signal corresponding to lamp intensity will vary considerably at different parts of the arrays or rows of elements present. This irregularity is caused by several factors, of which the following are mentioned by way of example: Different elements in the rows will have different sensitivity to light; lamp defects caused by powder distribution within the lamp will also influence the result; the short beam path and problems associated with wide-angle lenses of scanning optics are also contributory factors. Furthermore, the colour distribution and luminensce of a lamp will change with temperature and age. Further variations occur with variations resulting from the influence of temperature on electronic components, primarily on the rows of conversion elements. Variations will also occur in dependence on the constructional tolerances of the analogue components present. It must be possible to detect marks reliably and effectively, in spite of all the component-based variations that may occur.
As with all detection systems of the kind referred to here, there is found a point at which an increase in sensitivity will result in erroneous detection, due to noise occurring in the signals and in the system as a whole. One example of such problem-creating phenomena is manifested in the fact that the whiteness of the paper, the gray scale, is uneven with each individual coupon and varies from coupon to coupon, and that the marks are not totally black, but that the gray scale of the marks will vary within each individual mark and also possibly between different marks. The present system is particularly intended for detecting hand-made or printed marks on standard white paper, and where noise in the measuring signals is caused, inter alia, by the aforesaid variations.
It is known in the prior art that the presence of a mark can be established by applying a light-intensity set-point value, such that light-intensity below a given level will indicate the presence of a mark. This method places great limitations on suitable sensitivities and has a poor reliability. Some improvement can be made, by using set-point values which constitute the mean values of measurement values obtained from earlier measuring points.